1. Nitrogen cycling in low oxygen regions: how do autonomous, sustained observations factor into the debate on the role of classical denitrification versus anammox
Ken Johnson
MBARI

2. Where does NO3- go?

3. Free energy change (ΔG° kJ/4 e-)
Oxic Respiration:
O2 + ½C2H3O2-  HCO3- + ½ H+
Denitrification:
4/5NO3- + ½C2H3O2- + 3/10H+  2/5N2 + HCO3-
+ 2/5H2O
DNRA:
½NO3- + ½C2H3O2- + 1/2H+ + 1/2H2O  1/2NH4+
+ HCO3-
Anammox:
NH4+ + NO2-  N2 + 2H2O

5. Mean Conc
Input/ Loss
Process
Residence Time
PO43-
2 µmol/L
2x1010 mol/y
River input (Paytan & McLaughlin 2009)
100,000 years
NO3-
30 µmol/L
230 Tg/y x 14 g/mol
Global denitrification (Devries et al 2012)
2,500 years
Ocean volume 1.4x1021 Liters
Residence time = time required to add (or remove) the amount now present in the ocean.

11. Eastern Tropical North Pacific (ETNP)
6960, 7558
Arabian Sea
6381

14. Rates measured with incubations using 15N labeled substrates.

16. Arabian Sea
Denitrification dominates
Eastern Tropical South Pacific
Anammox dominates

17. Anammox rate
Denitrification rate

21. To get N*, we need PO43-, which we don’t measure!
(PO43-) = (PO43-)preformed + AOU * 1/150
(PO43-)preformed = x T x T x Longitude x Latitude

22. N* at 200 m (Gruber and Sarmiento, 1997)

23. Float 6960
ETNP

24. Float 7558
ETNP

25. Float 6381
Arabian Sea

27. Arabian Sea

29. Arabian Sea
Denitrification dominates
Eastern Tropical South Pacific
Anammox dominates

30. Anammox rate (mmol N/m2/d)
Anammox rate (mmol N/m2/d). Dashed line is region with high NO2- (Naqvi, 1991).

31. Arabian Sea -5 µmol/L/ 45 days = 50 nmol N2/L/d -5 µmol/L/
45 days x 200 m =
22 mmol/m2/d

32. Float 6960
ETNP

33. 5 µmol/L is a typical deficit in the N* minima
If they have a time scale of about 2 weeks
5 µmol/L /14 d = 14 nmol/L/h

34. Anammox rate
Denitrification rate

35. Physics has a big influence on observations (Float 7558 ETNP, 220 to 240 m)

37. Arabian Sea -5 µmol/L/ 45 days = 50 nmol N2/L/d -5 µmol/L/
45 days x 200 m =
22 mmol/m2/d

41. The N-Cycle is highly perturbed
The N-Cycle is highly perturbed. Human production of fixed nitrogen (nitrate, ammonia, urea) now exceeds the natural rate on land.
The Economist, May 26, 2011

42. Norman Borlaug, father of the “Green Revolution” and 1970 Nobel Peace Prize winner -- “the application of low-cost nitrogen derived from synthetic ammonia, has become an indispensable component of modern agricultural production (nearly 80 million tonnes consumed annually). It is estimated that 40% of today’s 6 billion people are alive, thanks to the Haber-Bosch process of synthesizing ammonia” (Borlaug, 2000).
The Borlaug “Hypothesis”: Higher agricultural yields are critical to preventing further deforestation – (but more generally high yields can limit NET environmental impact because less land is converted to agriculture, my interpretation).

46. Distance from Coast (km) Number of Papers % Negative O2 Trend
0-30 41
70.7
30-100 19
68.4
100+ 40
77.5
Published time series
Distance from Coast (km)
Number of Sites
% Negative O2 Trend
0-30
424
64.2
30-100
632
57.6
100+
2330
49.1
Analyze entire data set (all time series)